The beginning of a "generation" marks the official first episode of it's main region/league. So any Pokemon from that generation that appeared ANYWHERE in the anime (even in a fantasy or flashback) before said episode, including in movies and specials. For reference, Gen I includes movies 1 and 2, Gen II includes movies 3-5, Gen III includes movies 6-9, Gen IV includes movies 10-13, and Gen V movies 14-16. Included up to Gen VI. In case of confusion, Pokemon are listed in Pokedex order, not debut order.